The present invention provides a process for reducing the roughness from the surfaces formed in photoresists after a relief image is patterned and developed in the photoresist. In particular, the process involves exposing the patterned photoresist to a vapor to reduce roughness from the exposed surfaces of the photoresists.
In the course of manufacturing integrated circuits, a semiconductor wafer is typically coated with a photoresist layer. Generally, the photoresists used for this layer are organic photosensitive films used for the transfer of images and are typically exposed through a patterned photomask to a source of activating radiation such as ultraviolet light to form a latent image. The photomask has areas opaque and transparent to the activating radiation such that the photomask image is transferred to the photoresist layer. The latent image is then developed to yield a positive or negative relief image that is identical to or very similar to the photomask pattern. The photoresist layer may be positive or negative acting depending on the photoresist chemistry and developers chosen. The wafer is then further processed until the desired integrated circuit device is fabricated. The processing may be carried out according to a number of processing steps, including etching, doping and the like. Each module of the process typically requires the use of at least one patterned photoresist layer to transfer the desired properties and designs to the integrated circuit.
The relentless drive towards smaller sub-micron feature sizes in order that more and faster components may be included in a chip of a given size has created numerous technical challenges for microlithographic processing. The technical challenges include developing new materials and processes capable of accurately and reproducibly creating the desired small lithographic structures. Utilizing smaller wavelengths of radiation have made it possible to resolve smaller features. However, in order to transfer those small features into the integrated circuit it is necessary to make modifications to the photoresist chemistry.
Traditional I-line resists, based on novolac resins, have functioned well for critical dimension feature sizes down to 0.35 um. However, novolac resins are not suitable for use at the smaller wavelengths since these materials strongly absorb radiation from the wavelengths used below 365 nm. As such, in order to ensure the requisite optical transparency, new materials, such as poly(hydroxystyrene) and polyacrylates, are generally used as resins for the shorter wavelengths.
DUV photoresists resists are optimized for exposure to radiation having a wavelength at about 248 nm. DUV resists are generally based on a chemical amplification mechanism because it offers high contrast, good sensitivity and demonstrated ability to resolve patterns at 0.25 um and smaller. DUV photoresist formulations typically include a resin and a photoacid generator (PAG). Commonly used resins include modified polyvinyl phenol polymers or polyvinylphenol/acrylate copolymers in which the phenol or carboxylate groups are partially xe2x80x9cblockedxe2x80x9d or protected by moieties that can be chemically cleaved. As previously mentioned, these systems are typically based on an acid catalyzed mechanism wherein a strong acid is generated upon exposure to a photoacid generator compound present in the photoresist formulation to activating energy. The acid catalytically and chemically cleaves the protected group. As a result, a dissolution differential then exists between exposed (deprotected polymer) and unexposed (protected polymer) regions. It is this catalytic mechanism that is primarily responsible for the high sensitivity of these systems.
Photoresists optimized for exposure to radiation at 193 nm employ a variety of other protected polymers that necessarily contain few or no aromatic groups due to the strong absorption of 193 nm light to aromatic compounds. For example, some of these photoresists are based on acrylate polymers having various non-aromatic functional groups. Similar to DUV photoresists, these resists also utilize an amplification mechanism. The PAG chemistry of 193 nm resists is essentially the same as that of DUV resists.
At feature sizes below about 0.10 um it is anticipated that 157 nm wavelength light will be used for imaging. Photoimaging of these types of resists will likely require a vacuum since oxygen is known to strongly absorb at this wavelength. As such, a new generation of chemically amplified photoresist polymers will have to be designed to meet the challenging demands of lithography at this wavelength.
Inherent to all photoresists lithographically patterned is a phenomenon known as edge roughness, nanoedge roughness, or line edge roughness. Edge roughness refers to the irregularities or coarseness present on the surfaces of the photoresist after patterning and development. The greatest severity of edge roughness is typically observed on the sidewalls of the patterned photoresist and has a significant impact on the critical dimension budget. Edge roughness becomes increasingly important as one transitions to smaller feature sizes. For example, the critical dimension error budget targeted by lithographers for 100 nm lines is xc2x17 nm. If sidewall roughness for a 100 nm line causes variations of only 4 nm on either side of the targeted edge, then over half the critical dimension (CD) error budget has been used up. Edge roughness thus consumes an increasingly larger portion of the CD budget as the critical dimension of the feature shrinks. Accurate transfer of the roughness into the subsequently etched pattern will contribute to unacceptable variations in electrical performance.
There has been a great deal of effort directed towards understanding the root causes of edge roughness. The cause of edge roughness can be attributed to numerous factors. For example, edge roughness can be attributed to inadequate mask quality, poor resist performance, poor aerial image contrast near the limits of optical resolution, the plasma etch process and numerous other sources recognized by those skilled in the art. One particular class of edge roughness is associated with top surface imaging (TSI) processes based on selective silylation of the exposed resist film. Well-known problems with this approach include poor silylation contrast of the resists and low glass transition temperature, and hence higher mobility of the silylated polymer. However, in most cases other than TSI systems, edge roughness can be attributed to characteristics of the resist itself or to the lithographic processing of the image, i.e., post-exposure bake and developing conditions.
Another causal factor contributing to edge roughness is excessive acid diffusion generated upon exposure of the photoacid generator to the activating radiation at the edges of the latent image pattern, which may cause partial deprotection of the resist polymer beyond the edges. This is the main reason that so much effort has been directed at PAG design and understanding the kinetics of the deprotection reaction. Post-exposure bake conditions which drive diffusion of the photogenerated acid, can therefore have a very significant effect on edge roughness.
Another cause of edge roughness is believed to be uneven dissolution of the resist during development. Developer process conditions have been found to have a large influence on edge roughness with different effects on isolated and dense features often resulting. Several researchers have attributed this type of edge roughness to the resist polymer composition and non-homogeneous regions formed during lithographic patterning and development. For example, researchers have determined that the formation of polymer clusters during exposure and post expose bake (PEB) leads to edge roughness due to the differing solubility of the clusters and non-clustered polymers (T. Ha et al., Proc. SPIE, vol. 3999, 66 (2000)). Similarly, research has indicated that for some resists, phase separation of protected and de-protected polymers significantly enhances edge roughness (Q. Lin et al., Proc. SPIE, vol. 3999, 23 (2000)). Research has also revealed a strong dependence of edge roughness on resist polymer molecular weight and polydispersity (T. Yoshimura et al., Jpn. J. Appl. Phys., vol. 32, 6065 (1993)). More recently, researchers have discovered that not only these properties of the polymer, but also monomer ratio and the nature of the PAG influence edge roughness (S. Masuda et al., Proc. SPIE, vol. 3999, 25 (2000)).
Clearly, as noted, edge roughness can result from the complex interplay of a variety of factors. The demands on the resist formulator and lithographers to optimize several important properties simultaneously, such as resolution, sensitivity, environmental stability, etch resistance, and line edge roughness, inevitably result in compromising some properties at the expense of others. There clearly exists a great need for post lithography processes that could uniformly address all the causal factors for edge roughness.
Annealing the patterned photoresist features has been evaluated as a post lithography method to reduce edge roughness. In negative tone resists, annealing has been found to be effective for reducing edge roughness. However, it is reported that annealing deleteriously affects the feature and significantly changes the critical dimension size. (G. Reynolds, Ph.D. Dissertation, University of Wisconsin-Madison (1999)). Negative tone resists typically include a mechanism wherein the photoresist crosslinks upon exposure to activating energy and becomes insoluble in developer.
Annealing of chemically amplified, positive tone resists have also been evaluated. However, positive tone photoresists resist are more susceptible to flow deformation in an annealing process. Furthermore, thermal deprotection of these resists can occur at higher temperatures; causing release of the volatile blocking groups and concomitant shrinkage of the resist features.
Thus, there exists a need for a process for reducing the degree of edge roughness in patterned resist features without significantly affecting the critical dimensions of the feature. It is desirable that the process be extendable to those photoresists used to pattern features less than 250 nm, i.e., photoresists optimized and sensitive for exposure to activating radiation at wavelengths of 248 nm, 193 nm, 157 nm and the like. Moreover, it is desirable that the process be amenable to reducing edge roughness in positive tone photoresists as well as negative tone photoresists. Importantly, the process should be economical, easily implemented and not dependent upon the source or causal factors of the edge roughness.
The present invention is generally directed to a process for reducing roughness from a surface of a patterned photoresist. The process includes exposing a substrate having the patterned photoresist thereon to a vapor, wherein the vapor penetrates into and/or reacts with the surface of the photoresist. The substrate having the patterned photoresist thereon is then heated to a temperature and for a time sufficient to cause the surface of the photoresist to flow and/or react with the surface wherein the surface roughness decreases. Optionally, the substrate is exposed to radiation during the process to increase the etch resistance of the photoresist and/or facilitate the reaction of the vapor with the surface of the photoresist. The invention is especially suitable for use with those photoresists used for imaging feature sizes less than about 250 nm.
In one embodiment, the process includes exposing a substrate having a patterned photoresist thereon to a vapor, wherein the vapor penetrates into the surface of the photoresist. The substrate is heated prior to, simultaneous with or after exposure of the vapor, to a temperature and for a time sufficient to cause the surface of the photoresist to flow wherein the surface roughness decreases. The vapor lowers a glass transition temperature at the surface of the photoresist relative to a glass transition temperature of a bulk of the photoresist that is free from exposure to the vapor. The vapor is selected from a material that is miscible with or at least partially miscible with at least one component in the photoresist. The temperature for heating the substrate is below a glass transition temperature for a bulk of the photoresist wherein the bulk of the photoresist is free from exposure to the vapor.
Optionally, the process further includes exposing the photoresist to activating radiation for a time and energy sufficient to increase an etching resistance of the photoresist prior to, simultaneous with or subsequent to exposing the photoresist to the vapor. The radiation that is used to expose the photoresist has a wavelength in the ultraviolet range, x-ray range or includes electrons generated from an electron beam, or the like.
The vapor can be selected from a material that is reactive or nonreactive during the process. Preferably, the vapor is generated from a material with a boiling point less than about 200xc2x0 C. at standard atmospheric conditions. Examples of suitable non-reactive vapors include ketones and esters such as acetone, methyl ethyl ketone, butyl acetate, ethyl lactate and propylene glycol methyl ether acetate.
In another embodiment, the process for reducing edge roughness includes exposing a substrate having a patterned photoresist thereon to a reactive vapor. The reactive vapor diffuses into the surface of the photoresist and lowers the glass transition temperature at the surface exposed to the vapor. The substrate is heated to a temperature and for a time sufficient to cause the surface of the photoresist to flow wherein the surface roughness decreases. During the process, the patterned photoresist is exposed to an activating radiation prior to, simultaneous with or subsequent to exposing the substrate to the vapor wherein the activating energy reacts with the photoresist to generate a compound. The vapor reacts with the compound and adds mass to the photoresist. The compound is reactive with the vapor and is preferably, a free radical, a photoacid generator, a photobase generator, or the like.
The reactive vapor can be generated from a wide range of materials including but not limited, to vinyl ethers, epoxides, acrylonitriles, furans, coumarins, indenes, styrenes, acrylates, aryl halides, halosilanes, alkynes, alkenes, cyclic ethers and sulfur dioxide. The reactive vapor reacts with the photoresist during and/or after thermal flow of the surface to increase a glass transition temperature for the photoresist surface relative to a glass transition temperature of bulk photoresist wherein the bulk photoresist is free from exposure to the vapor. The reactive vapor is preferably selected to add mass to the photoresist. Preferably, the reactive vapor is generated from a liquid with a boiling point less than about 200xc2x0 C. at standard atmospheric conditions. The inventive process can be used in the manufacture of integrated circuits.